The main process sources for glycerin production have been high-pressure hydrolysis, transesterification of fats and oils to produce methyl ester and a glycerin byproduct, and saponification. Processing to produce refined glycerin of greater than 99.5% purity depends substantially on the type of impurities in the crude. Both pre-treatment and refining steps are typically necessary. With hydrolysis, the starting crude glycerin is likely to be nearly 85% water, hence multi-stage evaporators constructed of stainless steel are needed just for concentration. Other starting crude materials have high salt content and frequently employ thin-film distillation. A summary containing some common purification processes is provided in Ullman's Encyclopedia of Chemical Technology, Vol. A-12, pages 480–483.
As a result of the continuing rise in the cost of fossil fuels, there has been an increasing interest in biodiesel fuels as a supplement to or replacement for traditional fossil fuel sources. Biodiesel processing involves the production of alkyl esters of long chain fatty acids by reacting the source acid with a low molecular weight alcohol, such as methanol. A traditional process for manufacturing fatty acid alkyl esters involves the transesterification of triglycerides using methanol, in the presence of an alkali catalyst. In addition to the desired fatty acid alkyl esters, this process produces an effluent stream comprising glycerin (glycerol), excess alcohol, water, alkyl esters and a mixture of mono, di and triglycerides resulting from the transesterification step. The rapid worldwide expansion in the production of biodiesel fuel since 2000 is creating a fast growing supply of byproduct crude glycerin. This byproduct crude material may typically be 86–92 percent glycerol (glycerin) with methanol being the primary contaminant. The decanted glycerin is likely to be combined with a wash water stream from the biodiesel purification, and that aqueous stream can be expected to contain significant amount of methanol, glycerol, and sodium or potassium salts. Some common pretreatment steps are depicted in a process flow diagram for a biodiesel process found on page 50, FIG. 3.4, of the study report “Economic Feasibility of Producing Biodiesel in Tennessee”.
The methanol needs to be recovered for recycle to the biodiesel process, but water content should be relatively low. Evaporation of the methanol can be accomplished under vacuum conditions and utilizing falling film evaporators, but it is likely to be accompanied by a considerable amount of the water coming from the wash-water stream. Alternatively, as illustrated in the aforementioned flow diagram, the methanol can be separated from the water and glycerin using a distillation column. The column bottoms then undergo a phase separation wherein unwanted fatty matter is skimmed off. The aqueous glycerin stream then undergoes evaporation of much of the water in falling film evaporators in series to obtain about an 80–89 percent glycerin material. All of these pretreatment and recovery steps involve expensive custom-made equipment even before the refining begins. As shown in Ullman's, a practical refining scheme would be a wiped-film evaporator to produce a salt-free vapor stream feed that feeds into a vacuum distillation column. The product glycerin is removed as a side-draw and a recycle glycerin stream is taken off of one overhead condenser with an aqueous stream condensed subsequent to that.
Much of prior literature references on purification of glycerin, such as U.S. Pat. Nos. 2,615,924, 2,741,638, and 2,772,207, are aimed at dilute aqueous solutions such as those resulting from fermentation and hydrolysis. Furthermore, U.S. Pat. No. 2,234,400 describes purification after concentration to 80% comprising one or more steam distillations followed by treatment with activated carbon or the like to effect decolorization. Even then, ester-type impurities are still present.
U.S. Pat. No. 4,655,879 describes an approximately 10-step process comprising alkalizing in the presence of air, evaporating in a thin-film evaporator, redistillation of the residue, rectification in a low-pressure-drop column with reboiling in a falling-film evaporator, main product removal as a liquid sidestream, with carbon treatment for color removal.
In these references, methanol was not present, or not mentioned, as a constituent in the crude feed.
It would therefore be desirable to provide a low cost efficient process for purification of glycerin recovered from fatty acid alkyl ester processes, such as the manufacture of biodiesels. Such a process would also provide an efficient low cost means for recovering alcohol from the process for recycle to biodiesel production. In addition, it is desirable to utilize predominately commonly available equipment such as might be available from idle facilities or surplus equipment previously used for other purposes. The object of this invention is to provide a simple, low-cost process for purifying glycerin byproduct from biodiesel production integrated such as to efficiently recycle methanol and accept wash water from the biodiesel process.